WASHINGTON (CNN) -- The Rev. Rick Warren said Thursday that his upcoming forum with Sens. John McCain and Barack Obama will be aimed at asking them tough "heartland questions."

The Rev. Rick Warren appeared on CNN's "The Situation Room" on Thursday.

The author of the best-selling book "The Purpose-Driven Life" is to interview McCain and Obama on Saturday.

The candidates will appear together at Warren's 20,000-member Saddleback mega-church in southern California.

"Well, I'm a pastor, not a pundit," he told CNN's Suzanne Malveaux on Thursday's "Situation Room." "One of the things we're going to do is I'm going to ask identical questions to both candidates, which will be different.

"But I do want to know how they handle a crisis, because a lot of the things in the presidency often deal with things you don't know are going to happen, that we don't know will happen in the next four years. ... There are a lot of different things you can deal with in the life of a leader that will tell us more about the candidate than some of the typical questions," he said.

Warren said he won't endorse either candidate and will let his followers make up their own minds.

"I'm called as a pastor to shepherd all the flock, and I have in my church Democrats and Republicans and liberals and conservatives and moderates, and everybody in between," he said. "And my calling is to shepherd all those people, so I don't think it's appropriate for pastors to endorse."

Two weeks before the 2004 election, Warren sent out an e-mail to several hundred thousand pastors, essentially saying there were non-negotiable issues that Christians should consider when they go to the ballot box. Some of those issues included stem-cell research, abortion and gay rights.

Asked whether McCain has an advantage with evangelicals, Warren said he's not going to predict how the influential religious group will vote.

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He added, "I can tell you this: They're not a monolithic bloc, as the press frequently tries to make them out to be. I think that for many evangelicals, they're not convinced that either of these men is an evangelical. They may be believers in Christ, they may be Christian, but they want to know, for instance, their world view. And they want to hear it out."

Obama's positions in favor of abortion rights and same-sex civil unions have also created some tension among evangelical voters otherwise drawn to his candidacy.

But the Democrat, who is Christian, has made it a point to discuss his religion on the trail this year and launched an ambitious outreach effort targeting these voters, including private summits with pastors and a major campaign aimed at young evangelicals.

And Obama's evangelical supporters, including members of the new Matthew 25 political action committee, rallied around the Democrat in late June when Christian conservative James Dobson accused him of "deliberately distorting the traditional understanding of the Bible."

Polling suggests, however, that a majority of white evangelical voters are still backing McCain, though enthusiasm for the presumptive Republican presidential nominee among evangelicals is less than what it was for President Bush in 2004.

In a CNN poll conducted by Opinion Research Corp. on June 4-5, nearly two-thirds of white evangelical voters surveyed supported McCain, but 30 percent backed Obama. The poll's margin of error was plus or minus 6.5 percentage points.

By comparison, Bush received the support of 78 percent of evangelical voters in the 2004 election, according to exit polls.

But even as former GOP presidential candidate and Baptist minister Mike Huckabee -- who was thought to have locked up the evangelical vote given his background as a minister -- made a strong showing in the GOP primaries this year, McCain was pulling in a substantial number of evangelical votes.

McCain, who was raised an Episcopalian and now identifies himself as Baptist, rarely discusses his faith.

"I'm unashamed and unembarrassed about my deep faith in God. But I do not obviously try to impose my views on others," McCain said April 11.

Since then, the Arizona senator has met with many of the evangelical leaders who did not support his candidacy during the primary season. At a private meeting this summer, dozens of the movement's most prominent figures voted to support his campaign.

But by some accounts, their grass-roots efforts to rally the conservative Christian base have lagged recently.

False rumors that Obama is a Muslim threaten to undermine support from key voting blocs like evangelicals and Catholics.